U.S. patent number 4,465,940 [Application Number 06/368,787] was granted by the patent office on 1984-08-14 for electro-optical target detection.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Air. Invention is credited to Herman Graff, James D. Joseph.
United States Patent |
4,465,940 |
Graff , et al. |
August 14, 1984 |
Electro-optical target detection
Abstract
An apparatus directed to a signal processing circuit for
combining data from several spectral bands to enhance the
signal-to-background ratio of a target detection system. Two
spectral bands, with separate sensing arrays, receive analog
signals which are fed to separate multiplexer units. From the
multiplexers, the analog signals are applied to a comparator where
they are compared with each other and also separately applied to
shift registers where the signals are retained for further
processing. If, during comparison, the ratio of the two bands is
less than a threshold, there is an absence of cloud return and the
signals stored in both shift registers are combined in a final
output register. If the ratio of the two bands is more than a
threshold, the band having the most cloud return is not used and
the shift register containing the greater percentage of target
return is processed through the output register.
Inventors: |
Graff; Herman (Beverly Hills,
CA), Joseph; James D. (Oakdale, MN) |
Assignee: |
The United States of America as
represented by the Secretary of the Air (Washington,
DC)
|
Family
ID: |
23452737 |
Appl.
No.: |
06/368,787 |
Filed: |
April 15, 1982 |
Current U.S.
Class: |
348/25; 244/3.16;
250/208.1; 250/339.02; 250/339.14 |
Current CPC
Class: |
G01J
3/36 (20130101); G01J 3/2803 (20130101) |
Current International
Class: |
G01J
3/00 (20060101); H01J 040/14 (); G01J 001/00 ();
F41G 007/00 () |
Field of
Search: |
;244/3.16
;250/578,226,342,339,204 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3946382 |
March 1976 |
Kossiakoff et al. |
4001820 |
January 1977 |
Rosenbaum et al. |
4314151 |
February 1982 |
Suzuki et al. |
|
Primary Examiner: Nelms; David C.
Assistant Examiner: Austin; Ernest
Attorney, Agent or Firm: Singer; Donald J. Franz; Bernard
E.
Government Interests
RIGHTS OF THE GOVERNMENT
The invention described herein may be manufactured and used by or
for the Government of the United States for all governmental
purposes without the payment of any royalty.
Claims
We claim:
1. An apparatus for spatial target detection comprising:
a plurality of sensors, said plurality of sensors being divided
into at least two groups; each group having a multiplexing means
for combining signals from said sensors, and a separate memory
means for storing data representing the signals from its
multiplexer means;
data selection means operative to select a combination of data from
the separate memory means of the different groups if a ratio of the
data from the multiplexing means of the different groups is less
than a predetermined threshold, said data selection means being
alternatively operative to select data from only one memory means
if a ratio of the data from the multiplexing means is greater than
the threshold.
2. The apparatus of claim 1, wherein said memory means for storing
the signals from each multiplexer means includes analog CCD (Charge
Coupled Device) shift registers.
3. The apparatus of claim 2, wherein said data selection means
includes comparator means for comparing each group of data that has
been processed through the multiplexer means with each other and
with the threshold and generating a signal to an electronic
switching means for performing data selection.
4. The apparatus of claim 3, wherein said data selection means also
includes an additional memory for storing selected data.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to the field of electro-optics
and, more particularly, to electro-optics involved in target
detection techniques.
In developing systems to optically detect targets, problems are
encountered when the target appears amid a background of clouds. In
such situations, tradeoffs must be made in order to obtain a
sufficient signal-to-background ratio that will allow detection.
The signal-to-background ratio corresponds to a signal-to-noise
ratio in electrical devices. The bandwidth in which the system
operates should also be examined to maximize performance. Three
elements must be considered in bandwidth optimization. These are
(1) spectral characteristics of the target, (2) spectral
characteristics of the cloud background, and (3) atmospheric
transmission and spectral characteristics of the background in the
absence of clouds. The current infrared frequency band was chosen
primarily to provide adequate signal strength while minimizing the
background cloud return. The present art consists of a group of
sensors capable of a certain degree of signal-to-background
detection within a specified bandwidth feeding their information
into an electronic network for signal processing.
One apparatus for the resolution of targets in the presence of
clutter is described in U.S. Pat. No. 3,946,382. Another apparatus
for identifying a target by recognizing its radar signature and
separating this signature from background clutter is taught in U.S.
Pat. No. 4,001,820. Both of these apparatus are specifically
directed to radar signals and both involve a comparison with a
threshold. The present invention on the other hand relates to video
signals in general and involves more than one spectral
bandwidth.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a new and
improved signal processing scheme which significantly enhances the
signal-to-background ratio of a new sensor system for target
detection purposes.
According to the invention, two spectral sub-bands are used with
separate sensing arrays to receive analog signals from each
sub-band. The signals are coupled through shift registers and
multiplexers to a comparator where they are compared with each
other. If the ratio of the sub-bands is less than a predetermined
threshold, there is an absence of cloud return with the target
return and if the ratio is greater than the threshold, there is
cloud return with the target return. When there is an absence of
cloud return, the signals from both sub-bands are combined to
produce the output signal. When a cloud return is present, only the
signal from the sub-band having the greater percentage of target
return is processed.
A feature of the invention is the provision of two spectral
sub-bands with separate sensing arrays combined with digital logic
for comparing and selecting the optimum signal based upon the
amount of cloud return.
DESCRIPTION OF THE DRAWING
The single FIGURE is a schematic drawing of one embodiment of the
target detection circuit according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
The invention has many applications. One specific application which
demonstrates the principles of the invention is the automatic
separation of snow returns from cloud returns as viewed by a sensor
on a satellite. In this case, snow is the "target of interest" and
is highly reflective in the visible wavelength and poorly
reflective in the near infrared. In contrast, clouds are reflective
in both the visible and the near infrared.
A recent study has demonstrated that the ratio of the reflectance
of the near infrared wavelength to the reflectance in the visible
wavelength for moderately transmissive atomspheres will have a
large range depending on whether there are no clouds, waterclouds
or snow in the field of view. As the atmosphere becomes less
transmissive the ratio range decreases but a distinct separation
can be made between the various returns once one determines the
transmittance of the atmosphere. In moderately transmissive
atmospheres the ratio of near infrared reflectance to visible
reflectance is very low if there is snow in the field of view,
relatively high is there are water clouds in the field-of-view, and
intermediate if there are ice clouds in the field-of-view. This
study demonstrated the major phenomenological principle of this
invention, i.e., multi-spectral information when properly processed
can be used to separate targets of interest from "clutter" returns.
In this case cloud returns are considered "clutter" and snow is
considered the target.
The above illustration is complicated by the fact that the target
of interest, snow, is on the ground. Thus the opacity of the
atmosphere must be measured by observing the absolute reflectivity
of ground returns taking into account a knowledge of the type of
ground being viewed and the known solar reflectance angle.
In a case where the target of interest is sufficiently above the
ground so that the atmosphere above the target is essentially
always high to moderately transmissive one can avoid the
complication of calculating the opacity and use the simple
comparator circuits of this invention.
Consider the application of separating ice clouds from water clouds
in which the ice cloud is the target of interest. Assume the ice
clouds are being observed at a high solar elevation angle so as to
record ice cloud returns as close as possible to the threshold
noise level of the sensor. Under these circumstances one would like
to gather all the reflected photons possible and sum the reflected
photons in the visible wavelength band and the near infrared band
if there were no water clouds in the field-of-view.
If there were water clouds in the field of view one would still
like to measure the ice cloud above the water cloud but would
realize that the measurement in the visible wavelength would be
totally misleading so one would comprise and measure the
reflectance in the near infrared only as the best measurement under
the circumstances.
The advantage of this approach is that it permits viewing of dim
returns from ice clouds when there are no complicating water clouds
in the field-of-view and preserves partial information when water
clouds are present.
Also note that the ratio of near infrared to visible reflectivity
is lower for ice clouds than water clouds. Thus by applying a
threshold level to the ratio one can conclude that returns above
the threshold are probably water clouds and the best chance to
obtain ice cloud information is to examine the near infrared band
level. If the ratio threshold is not achieved the cloud is probably
an ice cloud and one would be interested in the photons reflected
both in the visible and the infrared.
Turning to the application of detecting man made objects in the
atmosphere above the clouds it is clear that the above principles
of cloud detection could be used to determine whether or not a
cloud was in the field-of-view. In the absence of clouds a broad
spectral band could be selected to collect the bulk of the target
emitted/reflected photons. This broad spectral band is achieved by
summing two spectral bands. In the presence of clouds the spectral
band could be narrowed to that portion of the spectral band where
target energy is relatively greater than the cloud reflected.
energy. This narrowed band consists of one of the two spectral
bands.
In the FIGURE, two groups of detectors 12 and 14 respond to signals
in two bands and route their analog signal into a processing
signal. One group of detectors 12 corresponds to sub-band A while
another group of detectors 14 corresponds to sub-band B. Each group
of detectors consists of a string of cells connected to a common
bus. Each cell contains more than one photoconductive or
photovoltaic detector typically a lead sulfide or mercury-cadmium
telluride detector, whose current varies with the amount of ambient
light. In a typical operation environment, a mirror (not shown)
faces a target or scene and reflects this view onto the groups of
detectors. The mirror may be mounted in such a fashion that it
rotates, thereby causing the target or scene to be scanned. This
situation is assumed to exist in the FIGURE. The target would first
be seen by the detectors of group 12 and second by the detectors of
group 14. The analog signals from each group of detectors are fed
into two multiplexer units, 16 and 18, corresponding to sub-bands A
and B, respectively. Delaying and adding, if desired, could be
performed on the analog signals before entry into the multiplexer
units. After multiplexing, a signal series output stream will be
available for each of the two spectral sub-bands. Each serial
output data stream is transported into separate CCD (Charge Coupled
Device) analog shift registers, 20 and 22. Comparison of the
outputs of sub-bands A and B reveals that the ratio of the two is
larger for a cloud return than for a target return. If the ratio of
sub-bands A:B is greater than a predetermined threshold level, the
presence of a cloud return is indicated. Threshold level can be
determined by a comparator 24 connected between the output analog
signals from the multiplexer units. If the ratio of sub-bands A:B
is greater than threshold, cloud return is present in sub-band A
and only the data from sub-band B will be transmitted for further
processing. On the other hand, if the ratio of sub-bands A:B is
less than threshold, there is an absence of cloud return and the
data from both spectral bands will be transmitted for further
processing.
In operation, the output signal from the comparator 24 is fed into
a transistor 30, typically a MOSFET type, at the gate electrode.
This transistor has its source electrode connected to ground and
its drain electrode coupled to a voltage source 32. The drain
electrode also has its output connected to the gate of a second
transistor 34, also typically a MOSFET type. The source electrode
of this second transistor is connected to the output of shift
register 20, while the drain electrode output is fed into a summing
shift register 36. The output signal from shift register 22 is also
fed into summing shift register 36. If the comparison of the two
sub-bands is less than threshold, the comparator's output signal is
low and will not turn on transistor 30. Thus, the output of this
transistor will be the supply voltage which will turn on transistor
34, allowing the data from shift registers 20 and 22 to combine in
summing register 36. If the comparison of the two sub-bands is more
than threshold, the comparator's output signal is high and will
turn on transistor 30. Thus, the output of this transistor will be
pulled low (toward ground) which will turn off transistor 34,
allowing only the data in shift register 22 to pass to summing
register 36. The data contents of the summing register now
represents the true signal for signal processing purposes.
Generally, the total charge packet from this summing device would
be sampled using a Distributed Floating Gate Amplifier (DFGA) with
taps on the 4th and 8th stages. The appropriate tap to be used
depends on the results of the threshold test. Four stages are used
if both bands are retained and eight stages if only one band is
chosen. The selection of the proper amplification is necessary to
assure proper scaling of the analog signal before analog-to-digital
conversion and further processing is performed.
It should also be noted that the operation of the comparator 24
involves determining the ratio of sub-band A to sub-band B. The
need to perform actual division is obviated by adjusting the gains
of the two distributed floating-gate amplifiers prior to
thresholding in the comparator.
Thus, while preferred constructional features of the invention are
embodied in the structure illustrated herein, it is to be
understood that changes and variations may be made by the skilled
in the art without departing from the spirit and scope of the
invention. In particular, any amplifiers with adjustable gains are
suitable for scaling of data from sub-bands A and B. In addition,
the positions of the multiplexer and the amplifier may be
interchanged without affecting the concept. In a monolithic focal
plane array, this latter approach allows use of a single amplifier
for each channel.
* * * * *